This invention relates to lithographic printing plates. More particularly, it relates to an improved antibuckling apparatus for post-curing lithographic printing plates by exposure to high intensity radiation for short periods of time.
Lithographic printing plates are characterized by the fact that they have printing or image areas and non-printing or nonimage areas in the same plane, the image areas being oleophilic and hydrophobic and the nonimage areas being hydrophilic and oleophobic. In printing with such a lithographic plate, an oil-based ink adheres to the image areas but not to the nonimage areas. Although the ink may be transferred from the plate directly to a receptor surface using, for example, a direct rotary press, it is normally transferred from the plate to a blanket roll or cylinder and then to a receptor surface, the latter technique being termed offset printing.
Lithographic printing has been known and used for many years; and so-called "photolithography" has more recently become a widely used technique, replacing for many printing applications letterpress printing which requires the formation of a printing plate having the printing or image areas raised above the plate surface. The ability to form a lithographic plate using photographic techniques, rather than the laborious process of typesetting or the exacting process of forming raised durable, well-defined indicia in a relatively thick plate coating, has materially increased the scope and range of printing jobs performed by lithography and particularly by offset printing. With the advent of margin justifying word processing equipment, high-speed photography and improved lithographic plates, offset printing has come to be used even in the printing of newspapers, books and the like where large numbers of copies are run.
In photolithography a thin coating of a photosensitive material is adhered to a plate which is generally of metal, although a plastic or special paper may be used for some special, limited applications. The metal plate is normally aluminum or zinc, or a combination of metal layers such as electrolitically deposited chromium on copper which may or may not be backed up with another metal such as steel or zinc to contribute strength and dimensional stability. The coatings on these plates are photosensitive in that upon exposure to radiation, e.g. light through the transparent areas of a photographic negative, they become insolubilized making it possible to remove the areas not so exposed with a suitable solvent to expose the plate surface. Thus the insolubilized areas become the image areas while the exposed plate areas become the nonimage areas.
The resulting plates usually retain residual radiation-sensitivity due to the relatively low doses of radiation applied during image-wise exposure. Among the disadvantages of plates prepared by this method are short press lives and low image durability.
The above problems have been obviated, in the past, by returning the image-wise exposed and developed plates to the imaging appartus for additional exposures to the imaging radiation for longer periods of time. This procedure has, inter alia, the disadvantage of tieing up the expensive, specialized imaging apparatus unnecessarily.
In my co-pending application Ser. No. 178,657, filed Aug. 15, 1980 now U.S. Pat. No. 4,326,018 there is described a method for overcoming the above disadvantages by post-exposing the developed plate to high intensity radiation for short periods of time. However, during such irradiation the temperature of the plate is greatly increased causing it to buckle, and the plate might jam and not be able to move further or the plate might contact the irradiation assembly which could damage that assembly or the plate.
U.S. Pat. No. 3,930,318, issued Jan. 6, 1976, describes a UV curing machine in which the radiation-heat induced buckling of the plate is minimized by a series of wires passing through the machine and maintained under tension. The wires pass through the space between the UV source and the plate, and portions of the presensitized plates directly under the wires are not exposed to radiation and, therefore, are not post-cured, thereby providing potential defects having the disadvantages described above.